1. Field of the Invention
The present invention generally relates to methods for determining the position of the armature of a solenoid. In particular, a method where the back-emf waveform generated by a current pulse applied to the solenoid coil is measured to determine the location of the armature.
2. Description of Related Art
A solenoid is a coil of insulated or enameled wire usually wound on a rod-shaped form, known as the armature, which produces a magnetic field when a current is passed through it. Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. The term solenoid is often used in reference to a magnet designed to produce a uniform magnetic field in a volume of space.
In a solenoid, the core material is usually ferromagnetic, meaning that it concentrates magnetic lines of flux. This increases the inductance of the coil far beyond the inductance obtainable with an air-core coil of the same dimensions and the same number of turns. When current flows in the coil, most of the resulting magnetic flux exists within the core material. Some flux appears outside the coil near the ends of the core; a small amount of flux also appears outside the coil and off to the side.
The inductance of a solenoid is the ratio of the magnetic flux to the current. This is more accurately referred to as the self-inductance of the circuit. The quantitative definition of inductance is L=Φ/i. When a solenoid armature moves relative to the solenoid coil, a change in self-inductance occurs.
In the field of engineering, the term solenoid may refer to a variety of transducer devices that convert energy into linear motion. The term is also used to refer to a solenoid valve, which is an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch. A solenoid switch is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch.
Solenoid valves are the most frequently used control elements in fluidics. Their tasks are to shut off, release, dose, distribute or mix fluids. Solenoids offer fast and safe switching, high reliability, long service life, good medium compatibility of the materials used, low control power and compact design
Because of these properties, solenoid valves are found in many application areas. Examples of these include an automobile starter solenoid or a linear solenoid, which is an electromechanical solenoid. Electromechanical solenoids are commonly seen in electronic paintball markers, dot matrix printers and fuel injectors. Pneumatic solenoid valves are commonly used to control pistons or other linear actuators. Hydraulic solenoid valves are used to control the flow of oil to rams or actuators to bend sheers of titanium in aerospace manufacturing, irrigation systems and the flow of fluid through an automatic transmission. In addition, solenoids can be used to provide for locking mechanisms, electrical switching systems, and other related devices.
One common use for a solenoid is to provide for a sample selector. In this use the solenoid can serve to draw a small portion of a larger sample into a testing area, The movement of the plunger/armature serves to essentially pull a portion of the sample into the testing area by its movement either by physically moving the sample by the movement of the plunger/armature, or by directing a larger scale piece by such movement. Such a use can provide for smaller electromechanical sampling devices and can be useful in a number of industries. It should also be noted that besides the plunger-type actuator which is used is in the above described sample selector, and most frequently in electromechanical systems in general, pivoted-actuators and rocker actuators are also used.
Regardless of how a solenoid is used, it is often desirable or necessary to determine the position of a mechanism that is operated by a solenoid or the location of a solenoid plunger/armature itself and systems have been developed in the industry for this purpose. Typically these systems use some form of position feedback comprising a variety of sensors that can verify that the mechanism has achieved the desired mechanical state after the solenoid has been energized. Generally, these systems are external to the solenoid and may comprise mechanical, optical or electrical systems added to the solenoid which can effectively determine the location of the solenoid plunger/armature or something attached to it. For example, the armature may break a laser beam in one position and not in another, or movement of the armature may cause a lever to move between two different, mechanically detectable positions.
In another example, there are systems in which two solenoid coils are used, one as a transmitter and one as a receiver. These secondary coils generally do not contain a motive element and are used for sensing only. Other systems use complex micro processor-controlled schemes to determine the location of the armature.
These position feedback devices add complexity and cost to the design where it is desirable to know the position of the solenoid's armature, or the mechanism attached to it. Further, in some uses of the solenoid, these position-sensing systems may be impractical as they can inhibit or interfere with the operation of mechanisms attached to the solenoid and may become a hindrance to product design.